The present invention relates generally to electrical overvoltage surge arresters of the type which include power varistors and relates more particularly, but not exclusively, to such arresters which have no power handling arcing gaps connected in series with the varistors and which have varistors of the zinc oxide type.
Overvoltage surge arresters can be considered to be high speed voltage sensitive switches which are normally in the open position and connected between an electrical system and ground or some other reference potential. Typically, they include an electrical series of one or more varistors and one or more arc gaps in an insulating housing. At higher voltages, there may be voltage grading resistors shunting the gaps and also certain other circuitry to afford better control of the arrester response to a surge.
When the arrester is in the steady state, essentially no current passes through it except for the steady state current through the grading resistors. A voltage surge in the system above a predetermined voltage, however, will cause the arc gaps to arc over and pass a large current to ground through the series power varistors, which are chosen to have a low resistance at such a voltage. As the system voltage returns to normal, the resistance of the power varistors rapidly increases until there is insufficient follow current through the arrester for the arcs to be maintained in the gaps and the arrester then clears to once again become an open switch. The gaps perform the functions of providing a sharp control of the switching function and of isolating the system voltage from the varistors in the steady state. This isolating is needed because the varistors may not have sufficient nonlinearity in their current-voltage characteristic for keeping the steady state current at the normal system voltage to a low enough value to prevent thermal damage to the arrester.
Recently developed varistors of the zinc oxide compound type have made it feasible to eliminate series arc gaps entirely from arresters. These varistors are often referred to as "high exponent" varistors. The "exponent" is the numerical exponent in the current-voltage relationship I = KV.sup.n for a varistor, where I is the current through the varistor, K is a constant, and V is the voltage across the varistor. Such high exponent varistors can have sufficient resistance at system voltage to pass a follow current which is not ordinarily significant, while nevertheless having a sufficiently rapid decreasing of resistance at predetermined surge voltages to afford close control of the arrester switching functions without any interposed gaps.
Varistors used in arresters are generally subject to a thermal runaway condition, and this is particularly true for high exponent varistors used without series arc gaps. The runaway condition is due to the tendency of the varistor at a set voltage to pass more and more current with increasing temperature.
An arrester without series gaps and with high exponent power varistors will pass a certain steady state current at the normal system voltage. The magnitude of this current will be affected by the manner in which heat generated by the current is dissipated from the arrester. If the steady state current is too high, then the temperature of the arrester will continue to rise and the current will increase until the arrester fails, since the temperature dependence of the varistor current is a higher order function than is the heat dissipation from the arrester. On the other hand, even if the steady-state current is well below the instability threshold, a series of surge currents might add so much energy to the varistors that they are unable to recover to the steady-state current and are thus pushed into a runaway condition.
The problem of thermal runaway in arresters has been recognized previously. Prior approaches to preventing runaway have concerned primarily improving the heat transfer between the varistors and the housing, so that the housing would dissipate enough heat to keep the varistors well below a temperature from which they might be pushed into runaway by any normally anticipated surge currents. Such a prior approach is described, for example, in U.S. Pat. No. 2,050,334 issued Aug. 11, 1936 to D. R. Kellogg. In Kellogg there is disclosed an arrester in which the space between the varistors and the porcelain housing is filled with a nonflammable insulator to improve the heat transfer to the housing. The insulator is cylindrical and is provided after the varistors have been fitted into the housing. Depending on the particular insulator form, it may be packed around varistors, embed them, or be inserted as a preformed cylinder.
A serious problem with the above prior approach is that any arcing across the varistors in a failure mode will be closely confined and will therefore result in a rapid generation of large volumes of gas. Such gas generation presents an increased liklihood of a violent explosion of the housing.